Okay, so it might seem difficult to be against an entire class of people—and especially against the biggest socio-economic class (the middle or bourgeois). But Barthes was suspicious of bourgeois culture exactly because it was the dominant culture and had the power to feed people ideas about how the world works. The middle class perspective was too big to fail—and too big to see through.
So in Mythologies, Barthes tried to uncover the myths or ideas of mainstream culture. He argues that the bourgeois person lives in a "false nature": they think that things are natural, when really they're man-made. (The Truman Show, anyone?)